To realize a super high-speed optical transmission system of 40 Gbit/s or more, adoption of the polarization multiplexing technique has attracted attention. The polarization multiplexing technique is one that focuses on a fact that there are two polarization states orthogonal to each other in the same wavelength, and is a method that transmits two pieces of independent signal information by using the two polarization states. For the optical transmission system using the polarization multiplexing technique, various control techniques have been proposed for improving the transmission quality of polarization multiplexed light (for example, refer to Japanese Laid-open Patent Publication No. 2002-344426, Japanese Laid-open Patent Publication No. 2003-338805, and Japanese Laid-open Patent Publication No. 2005-65027).
Incidentally in the abovementioned polarization multiplexed optical transmission system, it is known that a degradation amount of transmission characteristics occurring due to a fiber nonlinear effect and polarization mode dispersion (PMD) is different according to the pulse timing between orthogonal polarization components of the polarization multiplexed light (for example, refer to D. van den Borne, et al., “1.6-b/s/Hz Spectrally Efficient Transmission Over 1700 km of SSMF Using 40×85.6-Gb/s POLMUX-RZ-DQPSK”, Journal of Lightwave Technology, Vol. 25, No. 1, pp. 222-232, Jan. 2, 2007). Specifically, as illustrated in FIG. 11, in view of fiber nonlinear tolerance, time-interleaved polarization multiplexing in which a pulse between orthogonal polarization components of the polarization multiplexed light is shifted by half a bit, becomes an excellent transmission characteristic (right side in FIG. 11). On the other hand, in view of PMD tolerance, time-aligned polarization multiplexing in which pulse timing between orthogonal polarization components of the polarization multiplexed light become the same phase, becomes an excellent transmission characteristic (middle in FIG. 11). Accordingly, in order to obtain a desired transmission characteristic, the pulse timing needs to be set according to the state of the optical transmission system.
However, in the conventional polarization multiplexed optical transmission system, the pulse timing between the orthogonal polarization components is fixed by the initial setting of an apparatus that generates the polarization multiplexed light. Accordingly, even if a change of the system state such as; a time fluctuation of the PMD, a change of a wavelength path, or a change of a channel spacing occurs, the pulse timing between the orthogonal polarization components cannot be adjusted according to the change. Therefore, there is a problem in that the transmission characteristics significantly deteriorate due to the system state.
In order to make the pulse timing between the orthogonal polarization components alterable to deal with the abovementioned problem, for example, a configuration can be considered where a plurality of transmitters is prepared corresponding to different pulse timing, and the transmitters are switched and used corresponding to the system state. However, there is a disadvantage in that the apparatus becomes large, complicated, and expensive since a plurality of transmitters is provided. Moreover, in a configuration where the pulse timing between the orthogonal polarization components of one transmitter is manually changed according to the system state, a changing operation of the pulse timing takes a long time. Therefore when the system state changes at a high speed and frequently, it is difficult to correspond to such a change.